Testing the Tree of Life

by: Liebers Kate
Date: May 2008

The comb jelly cannot sting its prey, but it has certainly shocked its researchers.

Traditional evolutionary theory implies that more complicated creatures derive from simpler ones, yet recent research suggests that the organism at the first branching point in the animal evolutionary tree was the comb jelly  a more complex organism than the previously acknowledged animal ancestor, the sponge.

Lead Researcher Casey Dunn of Brown University clarified that although the study showed that comb jellies marked the first split in the animal evolutionary tree, the research does not imply that the comb jelly was the first animal.

"What it does do is help us infer what the structure of that common ancestor was," Dunn said, implying that it would have nerves and tissues.

Instead of the sponges marking the first fork in the animal evolution path, the latest research suggests two new hypotheses:

1. All multi-cellular animals derive from a simple animal without nerves and tissues and that such features evolved independently in animals such as the comb jelly.

2. The multi-cellular animals' common ancestor was more complicated than once thought, possessing the tissues, nerves, complex symmetry and many other characteristics previously suspected to have evolved later.

According to Dunn, the later is more likely.

"Although increasing complexity can arise as a result of evolution, evolution is not a reason, per se, that it should favor increasing complexity in all situations," said Dunn.

The benthic sponge could have been secondarily simplified if its sedentary lifestyle did not favor fancy features such as nerves or tissue, Dunn explained.

Despite the comb jelly revelation, the previously suggested evolutionary tree structure  for the most part  still stands.

National Science Foundation Program Director Patrick Herendeen pointed out that much of the research confirms the widely undisputed major animal groupings.

Herendeen said this provided confidence not only in the previous evolutionary tree structure, but also in the reliability of Dunn's team's methodology.

Thanks to advancements in sequencing technology, Dunn said the team was able apply an unprecedented amount of data to the research, providing a wider scope with which to observe a fuller diversity of living animals.

"Now when you look back at the data that place sponges as the earliest diverging animal, it actually isn't that strong," said Dunn. "It's just been thought for a long time that because they're simple that they split off early."

Positioning animals on the evolutionary tree can be a complicated task, especially when the animal has characteristics in common with multiple groups.

University of Hawaii research scientist Mark Martindale worked on the project's molecular biology operations. He said that when such a conflict arouse, molecular data "has more promise to have a more neutral or true representation of what the actual historical relationships between animals might be."

Nevertheless, Dunn, Herendeen and Martindale emphasized that more research is necessary.

"This is not the final answer in any way," Martindale said, "but it basically opens people's eyes to the fact that the traditional ways of viewing things may not be correct."

Herendeen said the technology Dunn's team used shows great promise. Martindale said he suspects that the major branches of different groups of animals will be more or less agreed upon in the next couple years.

Regarding the organization of the evolutionary tree, Dunn said the question is whether researchers will converge on an answer as more data is added.

The research of Dunn and his colleagues is published in the April 10 issue of Nature.